In a saucepan over low heat, gently warm 1 cup milk, the cocoa powder, maple syrup, and vanilla and whisk until just combined: do not boil! Warm it just enough so that it combines, but not so much that it’s hot or boiling. Immediately remove from the heat. Pour the mixture into a liquid measuring cup and add the remaining 1 cup milk. (Here you can refrigerate until serving, or go directly into the next step.)